Thanks to Schwilly for this one:
1) post picture on ts.com (like in the Rig Pics section, Team Potential Avatar thread)
2) right click on pic > properties
3) highlight and copy the url
4) go to your profile
5) you should have an option to change your avatar from one of the defaults to your own (check the little box)
6) paste url into the designated area
Cross-reference:
How to resize images.
Posting images.